Potential transformers are used to multiply or divide voltages precisely for the purpose of measurement or calibration. An ideal potential transformer 20 is illustrated schematically in FIG. 1. An ideal voltage source 22 is connected to transformer 20. The input voltage is vi(t) and the output voltage is vo(t). The output voltage vo(t) is proportional to the input voltage vi(t) by the turns ratio, n. Thus, vo(t)=nvi(t). The turns ratio n may be larger or smaller than one. For n larger than one, the transformer is a step-up transformer. For n less than one, the transformer is a step-down transformer. Of course, ideal transformers 20 and voltage sources 22 do not exist. Real world, non-ideal transformers exhibit such phenomena as common mode signal injection, winding resistance, winding-to-winding capacitance, winding-to-electrostatic shield capacitance, turn-to-turn and layer-to-layer capacitance, core loss, and magnetizing inductance.
FIG. 2 illustrates a typical model for a non-ideal potential transformer 24 and voltage source 26. The transformer in the, model is an ideal 1:n transformer. An electrostatic shield 32 is illustrated between the primary and secondary windings 28, 30 to eliminate electrostatic coupling between the transformer's primary winding 28 and secondary winding 30. Rg models the resistance of the non-ideal voltage source 26. Rp models the resistance of the primary windings 28. Rs models the resistance of the secondary windings 30. Cp models the turn-to-turn or layer-to-layer capacitance associated with the primary windings 28. Cs models the turn-to-turn or layer-to-layer capacitance associated with the secondary windings 30. Csh1 models the winding 28-to-shield 32 capacitance associated with the primary windings 28. Csh2 models the winding 30-to-shield 32 capacitance associated with the secondary windings 30. Rc models the core loss associated with the transformer 24 core. Lm models the magnetizing inductance associated with the transformer 24 core. Ideal voltage source vc(t) models the voltage associated with common mode signal injection. It should be understood that Rg, Rp, Rs, Rc, Cp, Cs, Csh1, Csh2, and Lm are all lumped parameter approximations of what are actually distributed values.
As can be appreciated from FIG. 2, current flow through Rc, Cp, Cs, Csh1, Csh2, Lm, Rg and Rp causes errors in the output of the potential transformer 24. Additional error is caused by current flow in Cs, and Csh2, which induces additional voltage drop across Rs.